The present invention relates to a multicolor image forming apparatus, and in particular, to control of press-contact/separation of an image forming member of an image forming unit which forms a multicolor image such as a full color image by compounding monochromatic toner images each being yellow (Y), magenta (M), cyan (C) or black (B) in color with an intermediate transfer member, and to control of a polygon mirror in an exposure device in each image forming unit mentioned above, in a multicolor image forming apparatus.
In a prior art:
(1) An image forming member in an image forming unit which forms a toner image for each of Y, M, C and K and an intermediate transfer member move in a press-contact condition, then, toner images of respective colors are transferred onto the intermediate transfer member to be superimposed on it, and a color toner image is formed. The color image forming process in which the toner image is formed as mentioned above is performed via a preparation process, in which the image forming member of each image forming unit is subjected to press-contact with the intermediate transfer member, before the image forming process.
(2) In a laser image forming apparatus such as a laser printer whose light source is a laser diode, an image writing is performed by a scanning exposure on the image forming member that is conducted with a light beam from a laser light source deflected by a polygon mirror. A motor which drives the polygon mirror is a high speed motor having a speed of tens of thousands of rotations per minute, and the considerable amount of rush currents flow therethrough when it starts. Further, it takes a few seconds for rising up from the start of rotation of the polygon mirror to its constant speed rotation.
The followings are problems to be solved by the invention.
(1) Incidentally, most of the full color image forming apparatus representing the typical example of the multicolor image forming apparatus are used as an apparatus of forming a black and white image representing a monochromatic image, however, in a waiting condition of the conventional multicolor image forming apparatus, the press-contact condition of the image forming member with the intermediate transfer member has been established unconditionally. Therefore, the preparation time has been requested when a full color image forming process is started from the waiting condition, or when a black and white image forming process is started from the waiting condition. Due to this, there has been a problem that the time from pressing an image forming button or from sending an image formation starting order by an external device such as a personal computer to outputting a first sheet of image, that is, a first copy-out time or first print-out time (hereinafter referred to as FCOT) becomes longer.
In the waiting condition, for example, in the construction wherein the image forming member of the image forming unit for K image and the intermediate transfer member are set to be in press-contact condition, while other image forming bodies and the intermediate transfer member are set to be in non-press-contact condition, there is required a preparation process in which the image forming bodies for Y image, M image and C image and the intermediate transfer member are subjected to be in press-contact condition, before the full color image forming process, and accordingly, it takes ten-odd seconds for this process, which means long FCOT.
The first object of this invention is to provide a multicolor image forming apparatus having short FCOT, by solving the above-mentioned problem in the conventional multicolor image forming apparatus.
(2) In the conventional multicolor image forming apparatus such as, for example, the full color image forming apparatus, the polygon mirrors of the exposure devices for Y image, M image, C image and K image have been started at the same time. Due to this, a rush current becomes larger, and a power supply that can comply with the large rush currents is necessary, which has caused a cost increase.
The second object of this invention is to provide a low-cost image forming apparatus, by solving the above-mentioned problem in the conventional multicolor image forming apparatus.
(3) In the example of the full color image forming apparatus, when there is employed the starting method wherein the polygon mirrors of the exposure devices respectively for Y image, M image C image and K image are started in succession at intervals of time, the polygon mirror of the exposure device for K image is started lastly.
When the black and white image formation is performed by the conventional full color image forming apparatus, the polygon mirror for K image is started lastly from the order for the start of image formation, after the polygon mirrors for Y image, M image and C image are started, and therefore, the starting time for the polygon mirror for K image is delayed, which causes a problem of longer FCOT.
The third object of this invention is to provide a multicolor image forming apparatus having short FCOT, by solving the problem that FCOT becomes longer in the black and white image forming by the multicolor image forming apparatus.
The above-mentioned objects of the invention are attained by the following invention.
(1) A multicolor forming apparatus, which has therein a first image forming unit for a first image having an image forming member, an image forming means for forming a toner image on the image forming member, and a primary transfer means, a second image forming unit for a second image having an image forming member, an image forming means for forming a toner image on the image forming member, and a primary transfer means, an intermediate transfer member carrying a toner image that is transferred from the image forming member by the primary transfer means, and a secondary transfer means for transferring a toner image on the intermediate transfer member onto a recording material, and performs a multicolor image forming process in which all of the image forming bodies are subjected to press-contact with the intermediate transfer member to form an image or a monochromatic image forming process in which only the image forming member of the first image forming unit is subjected to press-contact with the intermediate transfer member to form an image, wherein there are provided a priority mode setting means by which a multicolor priority mode and a monochrome priority mode are set and a control means which controls so that all of the image forming bodies are subjected to press-contact with the intermediate body in an idling condition and a waiting condition, when the multicolor priority mode is set, and only the image forming member of the first image forming unit is subjected to press-contact with the intermediate transfer member in an idling condition and a waiting condition, when the monochrome priority mode is set.
(2) The multicolor image forming apparatus described in (1) wherein the primary transfer means is composed of a transfer roller to which a transfer voltage is applied, and the image forming member is press-contacted with and separated from the intermediate transfer member, by the displacement of the transfer roller.
(3) The multicolor image forming apparatus described in (1) or (2) wherein the control means controls the separation of the image forming member except the image forming member of the first image forming unit from the intermediate transfer member, when the waiting condition has been kept for the prescribed time period.
(4) A multicolor image forming apparatus wherein there are provided an exposure device for a first image and an exposure device for a second image having respectively a laser light source and a polygon mirror which deflects a light beam from the laser light source, and a control means which controls the starts of the exposure device for the first image and the exposure device for the second image so that they may start in succession at the time intervals.
(5) The multicolor image forming apparatus described in (4) wherein the control means makes the polygon mirror of the exposure device for the first image to start lastly when a first image is a multicolor image, and makes it to start first when a first image is a monochromatic image.
(6) A multicolor image forming apparatus having the exposure device for the first image and the exposure device for the second image having respectively a laser light source and a polygon mirror which deflects the beam from the laser light source, and a time setting means which sets an automatic shut-off time to shut off the polygon mirror, wherein the time setting means can set an automatic shut-off time of the polygon mirror of the exposure device for the first image and an automatic shut-off time of the polygon mirror of the exposure device for the second image to be different each other in terms of a value.